In the past, it has been necessary to obtain specimens or samples of various suspected materials which were believed to contain some percentage of uranium oxide ore and transport these specimens to an analysis site. It was then necessary to crush the sample into finely divided particles and pack these particles into a known volume sample container. The container, along with the unknown sample, was then subjected to an instrument which would read out the amount of radiation being emitted from the sample in order to determine the percentage of uranium ore present. Normally this analysis and readout of the sample would have to be performed in a laboratory usually located a considerable distance from the place where the sample was obtained. This arrangement is not only required with respect to mining of uranium at a remote mine site but in analyzing core samples which are taken from earth bores. The analysis of earth bores and geological strata has in the past been both a time-consuming and inaccurate method of analysis.
The primary inaccuracy occurs due to the fact that movement and time delay in making the actual analysis allows portions of the radioactive daughters of the original uranium element such as radon (gas) to be lost. Thus, it is necessary and desirable in order to improve this accuracy to perform the analysis in-situ and to obtain the results instantaneously to determine the exact percentage of uranium ore present.
This is especially true with respect to open strip mining of uranium ore where it is desirable and necessary to determine exactly where the highest percentage ores are located. Under present operations, ore material is stripped from the surface and transported by truck or conveyor to a location where the ore is graded and sorted so that the material containing an economical percentage of uranium ore will be retained for further processing. Those materials which do not have a sufficient quantity of uranium ore are dumped and eliminated from further processing. Where the mining operation is dealing with marginal ore grades, the most accurate method of determining ore percentage is mandantory. Thus, if an accurate instantaneous monitoring of ore grade is available, it will eliminate the necessity for stripping uneconomical material in the first place and for accurately determining the sorting of the once mined ore so that marginal ore can be economically processed.
The same type of situation occurs in earth bores during the geological investigation of uranium mineralization within the earth. Thus, in many cases, earth bores are used in geological investigations to determine the location of economical concentrations of uranium ore prior to establishing mining operations. As explained above, at present the core is taken from the bore holes and crushed in a laboratory, and subjected to individual sample analysis by means of a laboratory instrument.
This type of analysis, in the past, has been directed to alpha, beta, or gamma radiation detection and the use of various calculations or charts and tables to correlate the radiation readings of these various types of radiation to determine the quantity and type of materials that are present in the sample.
In order to be able to better understand the significance of the present invention, it is necessary to thoroughly understand the background concerning detection and determination of radio activity with respect to various isotopes of uranium.
As is well known, uranium 238, the most abundant naturally occurring isotope of uranium is radioactive. This is to say that the atomic nucleus of the uranium atom is unstable and has a finite probability of undergoing spontaneous radioactive decay to create the nucleus of another element. Thus, as can be seen by Table 1, uranium 238 decays to form thorium 234. Thorium 234 is also unstable and radioactive and further decays forming protactinium 234 which further decays to form radioactive uranium 234 and so on through the decay series until a stable nucleus, lead 206, is reached. The decay series for uranium 238,as illustrated in Table 1, can be divided into two essential groups; the uranium group which is comprised of the parent uranium 238 and its four daughters, and the radium group which consists of radium 226 and all subsequent daughters down to the formation of lead 206.
If an initially pure sample of uranium 238 is allowed to remain undisturbed in the ground for a long period of time, it will progressively decay and generate all of the nuclides shown in Table 1. Thus, a steady state or "equilibrium" concentration is reached at which the rate of formation of the radioactive nuclid is essentially the same as its rate of decay. This equilibrium concentration, at any given time, will be directly proportional to the original concentration of parent uranium.
Ideally, this equilibrium will be the status of natural deposits of uranium ore which are undisturbed. The analysis of uranium ore grade under such circumstances would be relatively simple and accurate. The disturbance of the uranium ore, however, causes a disequilibrium due to the loss of certain elements or daughters, causing an unblance between the uranium and radium groups. This can either be caused by leaching of some or all of the uranium group elements from the original deposit by means of ground water or by loss of the gaseous daughter radon 222 by subterranean migration or by disturbance from a mining or drilling process.
TABLE I ______________________________________ URANIUM DECAY SERIES ENERGY GROUP ELEMENT EMITTED ______________________________________ Uranium Group Uranium 238 .dwnarw. .alpha. Thorium 234 .dwnarw. .beta..gamma. Protactinium 234 .dwnarw. .beta. Uranium 234 .dwnarw. .alpha. Thorium 230 .dwnarw. .alpha. Radium Group Radium 226 .dwnarw. .alpha. Radon (gas) 222 .dwnarw. .alpha. Polonium 218 .dwnarw. .alpha. Lead 214 .dwnarw. .beta.,.gamma. Bismuth 214 .dwnarw. .beta., .gamma. Polonium 214 .dwnarw. .alpha. Lead 210 .dwnarw. .beta. Bismuth 210 .dwnarw. .beta. Polonium 210 .dwnarw. .alpha. Lead 206 ______________________________________
As can be seen from Table 1, each disintegration from one nuclide to the next lower in the series is accompanied by the emission of energy. This energy may take the form of an alpha particle (helium ion), a beta particle (electron) or gamma radiation (high energy photon). The radiometric analysis of these emissions can be performed by suitable detectors which are well known in the art. Since the rate of emission is proportional to the concentration of the emitting nuclides, the analysis can be made.
The counting of gamma radiation is the simplest and most commonly used technique for uranium ore analysis. In the past, scintillators or geiger-meuller counters have been used to detect the gamma radiation. However, this method is indirect since approximately 98% of the gamma activity of equilibrium uranium ore is produced by the radium group, notably the nuclides lead 214 and bismuth 214. What the gamma detectors acutally sense is the amount of lead 214 and bismuth 214 present in the ore sample. Under equilibrium conditions, the levels of these isotopes are directly proportional to uranium content, and gamma activity can be related directly to ore grade or percentage.
However, as is also well known, both lead 214 and bismuth 214 are short-lived daughters of radon 222. Hence ore disequilibrium caused either by uranium-radium group unbalance or by radon gas loss severely effects the gamma activity of the ore and interpretation of ore grade can be extremely inaccurate or misleading. If the disequilibrium is caused by the loss of radon 222 which has a half-life of only 3.8 days, the equilibrium conditions can be restored by means of the "closed can" gamma assay method which is well known. The disequilibrium resulting from uranium-radium unbalance is more serious however. The long-lived isotopes uranium 238, thorium 230, and radium 226 can return to equilibrium only after as much as 10.sup.4 to 10.sup.6 years of undisturbed decay. Under these circumstances, where disequilibrium has occurred from this unbalance, gamma only counting cannot be used to accurately determine ore grade.
With the present invention a beta-gamma method of accurately determining uranium ore grade is obtainable. As far as the beta radiation activity of uranium ore in equilibrium is concerned, 60% results from the radium group. The bulk of the remainder is generated by the decay of protactinium 234, the second daughter of uranium 238. Because of the short half-life (24 days) of its predecessor, thorium 234, and its own short half-life of 1.175 minutes, protactinium 234 is invariably in equilibrium with and hence proportional to the uranium content of natural ores. Thus, measurement of the beta activity of protactinium can provide a reliable means in combination with the gamma radiation reading for the accurate determination of uranium ore grade. The major problem concerned, however, is that the beta radiation activity of the uranium group must first be distinguished from that of the radium group.
Table 1 shows that there are six principle beta emitters in the uranium series. Suitable shielding or attenuation of a beta detector permits only the more energetic beta radiation of protactinium 234, lead 214, and bismuth 214 to be counted. Thus, under these circumstances, the count rate of the beta radiation is proportional to the combined concentrations of these three elements. It has already been shown that the bulk of gamma radiation activity of uranium ore is generated by the decay of lead 214 and bismuth 214 which in turn is comparable to the beta radiation from these elements. Suitable attenuation of gamma ray detection permits gamma rays from only these two nuclides to be monitored. Thus, the subtraction of the count rate of gamma radiation from the beta count rate essentially leaves the count rate of the beta emission caused by protactinium 234 which allows us to determine uranium ore grade directly. This is the basis of the beta-gamma method. As can be easily seen, the key to the use of this method of determining ore grade is by the use of proper attenuation of the beta and gamma radiation detectors. This is one of the important features of the Applicants' invention, and allows the direct determination of percent of uranium oxide (U.sub.3 O.sub.8) present in the analysis at the location site.
Although this is a simplistic explanation of the analysis process used in the present invention, it is intended to provide a background and understanding of the novel features of the Applicants' invention.